In view of global environmental protection, it is an important task today, for example, to increase automobile fuel efficiency. Accordingly, it is actively attempted to reduce the weight of the automotive body by strengthening the body material so that the thickness can be reduced. However, strengthening of steel sheet reduces the formability. A material having both high strength and high formability is desired.
Accordingly, a variety of multi phase steel sheets have been developed, such as ferrite-martensite dual phase steel (so-called dual phase steel) and so-called TRIP steel using the transformation induced plasticity of retained austenite. For example, Japanese Patent No. 1853389 discloses a high-strength steel sheet with low yield ratio superior in surface quality and bendability, produced under specific conditions of chemical composition, hot rolling, and annealing, and a method for manufacturing the same.
PCT Japanese Translation Patent Publication No. 2003-505604 discloses a method for manufacturing a high-strength steel sheet having superior mechanical properties by specifying the chemical composition, the amount of martensite, and the manufacturing process. Japanese Patent No. 3610883 discloses a method for manufacturing a steel sheet having a superior bendability by hot-rolling and annealing a steel having a specifying composition under specific conditions. Japanese Unexamined Patent Application Publication No. 11-61327 discloses a method for manufacturing a steel sheet giving crash safety to automobiles and having superior formability by specifying the volume fraction of martensitic phase and the martensite grain diameter and mechanical properties.
In addition, Japanese Unexamined Patent Application Publication No. 10-60593 discloses a steel sheet having superior stretch flangeability in which the chemical composition and phase structure and the hardness ratio of the phases are specified. Japanese Unexamined Patent Application Publication No. 7-11383 discloses a steel sheet exhibiting superior fatigue characteristics in which the chemical composition and phase structure, and the grain diameter and hardness ratio of the phases are specified.
The above-cited steel sheets may be galvanized to form a zinc coating to improve the corrosion resistance in practical use. In this instance, to ensure press formability, spot weldability, and paint adhesion, galvannealing is often applied in which the zinc coating is heat-treated to diffuse the Fe of the steel sheet into the coating. Following this trend, a variety of steel sheets are being developed.
For example, Japanese Patent No. 3527092 makes a proposal for galvanized steel sheet, and discloses that high-strength galvannealed steel sheet is improved in ductility and formability by specifying the chemical composition and the amount of retained austenite. Also, Japanese Unexamined Patent Application Publication No. 2003-213369 proposes a high-strength steel sheet, a high-strength galvanized steel sheet, and a high-strength galvannealed steel sheet produced by specifying the composition and the volume fraction and grain diameter of martensitic phase, and thus exhibiting superior stretch flangeability and crash resistance. Japanese Unexamined Patent Application Publication No. 2003-213370 proposes a high-strength steel sheet, a high-strength galvanized steel sheet, and a high-strength galvannealed steel sheet produced by specifying the chemical composition, the ferrite grain diameter and microstructure, and the volume fraction of martensitic phase, and thus exhibiting superior stretch flangeability, shape fixability, and crash resistance.
Japanese Patent Nos. 2862186, 2862187, 2761095 and 2761096 propose methods for manufacturing a steel sheet superior in ductility, stretch flangeability, and bendability by heat-treating a steel having a predetermined composition under specific conditions in a continuous galvanizing line. Japanese Unexamined Patent Application Publications Nos. 6-93340 and 6-108152 propose high-strength galvanized steel sheets whose stretch flangeability and bendability are improved by specifying the chemical composition and conditions of the galvanizing line, and methods and apparatuses for manufacturing the high-strength galvanized steel sheet.
Furthermore, Japanese Unexamined Patent Application Publication No. 2004-115843 proposes a steel sheet superior in shape fixability, coating adhesion, and ductility in view of wettability and quality of coating, produced by specifying the chemical composition, the amount of martensite, and the Fe content in the coating. Japanese Unexamined Patent Application Publication No. 2002-47535 proposes a steel sheet superior in ductility, flaking, and powdering, produced by specifying the chemical composition, the amount of retained austenite, and the Fe and Al contents in the coating.
Japanese Unexamined Patent Application Publication No. 2001-288550 proposes that, for a high Si, high Mn steel to ensure good appearance of the coating, a galvanized steel sheet without an uncoated portion be produced by controlling the high Si—Mn layer formed in the vicinity of the interface between the coating and the base steel to an appropriate state. Japanese Patent No. 3459500 proposes that a high-strength galvannealed steel sheet superior in form ability and coating adhesion be produced by specifying the chemical composition and the process conditions.
Unfortunately, the above known art has the following problems. For example, Japanese Patent No. 1853389 discloses that annealing is performed in a single phase region, followed by cooling to 400° C. at a rate of 6 to 20° C./s. However, the manufacture of a galvanized steel sheet requires taking coating adhesion into consideration. In addition, since cooling to 400° C. is performed to a temperature lower than or equal to the hot dip zinc pot temperature, the temperature must be increased before galvanization. It is thus difficult to manufacture a galvanized steel sheet in a continuous galvanizing line (CGL), which does not have any heating device upstream from the hot dip zinc pot.
PCT Japanese Translation Patent Publication No. 2003-505604, Japanese Patent No. 3610883, Japanese Unexamined Patent Application Publication Nos. 11-61327 and 7-11383, Japanese Patent No. 3527092 and Japanese Unexamined Patent Application Publication No. 2003-213370 do not consider stretch flangeability, and do not produce stable coating adhesion or stretch flangeability particularly when galvanization is applied. Japanese Unexamined Patent Application Publication No. 10-60593 proposes that the phase structure is turned into ferrite and bainite or pearlite to ensure a sufficient stretch flangeability. This technique however does not produce sufficient ductility. In Japanese Unexamined Patent Application Publication No. 2003-213369, the stretch flangeability and the crash resistance are enhanced by specifying the chemical composition and the martensite grain diameter and volume fraction. However, when a ferrite-martensite multi-phase structure is particularly galvanized, satisfying stretch flangeability cannot be produced stably.
Japanese Patent No. 2862186 produces superior ductility, but does not consider stretch flangeability or bendability. In contrast, Japanese Patent Nos. 2862187 and 2761095 produce superior stretch flangeability and bendability, but do not consider ductility. Japanese Patent No. 2761096 is intended to enhance the ductility and results in insufficient stretch flangeability, and is applied to only a limited portion. Japanese Unexamined Patent Application Publication Nos. 6-93340 and 6-108152 require that the martensite be produced by annealing during heat treatment in the galvanizing line, and accordingly require an apparatus for reheating after cooling to the Ms temperature or less.
Japanese Unexamined Patent Application Publication Nos. 2004-115843 and 2002-47535 can ensure wettability and coating adhesion, but do not consider stretch flangeability. In particular, in galvanization, coating adhesion or stretch flangeability is not stably exhibited. Japanese Unexamined Patent Application Publication No. 2001-288550 exhibits good appearance of the coating when a Si—Mn layer is formed at the grain boundary neat the interface, but does not sufficiently ensure mechanical properties. Mechanical properties are also significantly varied disadvantageously. Japanese Patent No. 3459500 proposes a steel sheet superior in formability and coating adhesion produced with a specific chemical composition under specific conditions. However, steel sheet superior in formability is not necessarily produced only by specifying the chemical composition and manufacturing conditions, and exhibits large variation in characteristics.
Accordingly, it could be helpful to overcome the above-described problems and to provide a high-strength galvanized steel sheet having a tensile strength of a level of about 590 MPa, including a coating with good appearance, and exhibiting superior formability.